1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to, a method for forming metal lines in a NAND flash memory device.
2. Discussion of Related Art
FIG. 1 is a cross-sectional diagram illustrating a conventional metal line structure of a NAND flash memory device. A conventional method for forming metal lines will now be explained with reference to FIG. 1.
Referring to FIG. 1, a first contact plug 14 exposing a conductive region in a predetermined region of a semiconductor substrate 10b including cell strings 10a is formed by patterning a first interlayer insulation film 12 on the whole surface of the semiconductor substrate 10b. 
A second contact plug 18 exposing a conductive region except the conductive region in which the first contact plug 14 has been formed is formed by patterning a first interlayer insulation film 12 and a second interlayer insulation film 16 over the resulting structure.
First metal lines 28a, 28b and 28c exposing a conductive region except the conductive region in which the first contact plug 14 and the second contact plug 18 have been formed are formed by forming an etch barrier film 20 and a third interlayer insulation film 22 over the resulting structure, and patterning the first, second and third interlayer insulation films 12, 16 and 22. Second metal lines 24 and 26 stacked on the first contact plug 14 and the second contact plug 18 are formed by patterning the second and first interlayer insulation films 16 and 12.
Third contact plugs 32a, 32b and 32c exposing the first metal lines 28a, 28b and 28c and the second metal lines 24 and 26 are formed by forming a fourth interlayer insulation film 30 over the resulting structure, and patterning the fourth interlayer insulation film 30.
However, the conventional method for forming the multi-layer line structure in the NAND flash memory device has the following disadvantages.
First, a contact hole is formed through the first, second and third interlayer insulation films during the process for forming the first metal lines. Therefore, an aspect ratio of the contact hole increases, and thus the contact hole is not efficiently filled.
Second, an ion implant process for increasing an impurity concentration which may be reduced in an impurity region after the process for forming the contact hole is performed three times, namely, after forming the contact hole for forming the first contact plug, after forming the second contact hole for forming the second contact plug, and after forming patterns for forming the first metal lines. In order to perform the ion implant process three times, a plurality of processes such as a masking operation are required to prevent ions from being implanted into other films. Accordingly, steps of the whole process are complicated.
Third, the first and second contact plugs and the first metal lines use different metal materials, to complicate the steps of the process.